Patent GB-942096-A (GB)
Title: Improvements in or relating to oriented films
Inventor: BUTEUX RICHARD HAROLD BARCLAY
IPC: B29C48/10
Filed: 1961-07-10
Abstract: 942,096. Formaldehyde polymer films. IMPERIAL CHEMICAL INDUSTRIES Ltd. June 29, 1962 [July 10, 1961], No. 24858/61. Heading B5B. A substantially uniformly thick asymmetrically oriented film of a linear high molecular weight formaldehyde polymer has a tensile break strength in the longitudinal direction of at least 40,000 p.s.i. and a ratio of tensile break strengths in longitudinal and transverse directions of more than 1.3 : 1. Such film is made by heating film having little or no transverse orientation to a temperature 22-8 C. below the optical melting-point of the polymer, and longitudinally stretching to a ratio of at least 4:1. Alternatively, a film previously stretched in both longitudinal and transverse directions may be re-drawn longitudinally at a temperature with 15 C., preferably within 8 C. of the optical melting-point, provided that the product of the two longitudinal stretch ratios is at least 4:1. Similarly a film previously stretched only in a transverse direction, preferably to a ratio of less than 4:1, may be redrawn longitudinally. The asymmetrically oriented film may also be made by simultaneously stretching an unoriented film in two directions at right angles, and to a greater extent longitudinally (stretch ratio at least 4: 1) than transversely, the film being cooled from the melt to a temperature close to but below, e.g. within 20 C. of, the optical melting point, and stretched as soon as possible, e.g. within 20 seconds, after cooling, before the film is fully crystallized. Such simultaneous but differential stretching may also be carried out on fully crystallized, unoriented film, at a temperature in the range 22-8  C. below the optical melting-point. The term " linear high molecular weight formaldehyde polymer" includes homopolymers of formaldehyde or a low molecular weight polymer of formaldehyde such as trioxane, or may include a random or block copolymer consisting of oxymethylene units with e.g. 0.1-15 mole per cent of other monomer units each containing 2 or more adjacent carbon atoms in the polymer chain. Suitable co-monomers for polymerizing with trioxane are, e.g. ethylene oxide, 1,3-dioxolane, butadiene paraldehyde, butadiene mono-oxide, styrene oxide, indene, coumarone, γ-propanolactone, ethylene carbonate, 1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, 1,2- propylene oxide, 1,2-butylene oxide diethylene glycol mono-formal, pentamethylene formal, neopentyl formal, pentaerythritol diformal, 1,3 - dioxep - 5 - ene, 2 - isopropyl - 1,3 - dioxep - 5 - ene, 2,2 - dimethyl - 1,3 - dioxep - 5 - ene, methacrolein dimer, xylylene glycol formal, 1,3 - dioxa - 6 - thiacyclo - octane and the cyclic formal of bis-(#-hydroxyethyl) sulphone. The polymers may be stabilized by small amounts of amines. The degree of polymerization is preferably such that the melt viscosity of the polymer at 190 C. at low shear stress is 10<SP>3</SP>-10<SP>6</SP> poises. In the apparatus of Fig. 1, a polymer of optical melting-point, e.g. 168 C., is melt extruded from die 2 in close proximity to sand-blasted, chromium-plated sizing die 3, which cools the tube to e.g. 155 C. to enable it to be gripped by V-section, silicone rubber surfaced caterpillar belts 5. The inner surface of the tube is cooled by air at about 140 C. issuing from air-bearing 4, which is supplied with heated air by pipe 10, and which also carries pipe 9 for supply of air for inflating the tube. The tube is then heated to e.g. 157- 158  C. by infra-red heaters 7 and stretched and hauled off by nip-rolls 8. In Fig. 2, molten polymer is supplied under pressure through pipe 11 to a chamber 12 and then to orifice 14, via die lands 13 surrounded by chambers 15, 15<SP>1</SP>, maintained at constant temperature by a boiling liquid. Cooling liquid is supplied through annular passages 16, 16<SP>1</SP>. The die is raised to operating temperature by electric heaters 17, 17<SP>1</SP>. Heat is retained by lagging 18. In Fig. 3, a tube 21 of polymer is melt extruded from die 22 in close proximity to cooled, sandblasted, chromium-plated sizing die 23. The tube, after passing through nip rolls 24, is heated to e.g. 155 C. by infra-red heaters 26. The tube is then stretched, and after being collapsed by guide rolls 27, is hauled off by nip rolls 28. A probe 25 supplies inflating gas between nip rolls 24, which have sponge-rubber grooves to receive the probe. The films have a volume resistivity of at least 10<SP>6</SP> ohms/cm. measured at 85 volts and 20  C., and a low water absorption, e.g. less than 0.5% w./w. Specification 787,479 is referred to.